1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modern radio frequency (RF) transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes, such as frequency shift keying (FSK) and phase shift keying (PSK), often in combination with Code Division Multiple Access (CDMA) communication. Some of these communication schemes, for example the 270.83 kbit/s binary Gaussian FSK employed in the GSM cellular telephony standard, have constant envelopes and the transmitter signal, sRF(t), may be represented mathematically assRF(t)=R cos(2πfct+θ(t)),   (1)where R denotes a constant amplitude, fc denotes the RF carrier frequency, and θ(t) denotes the information bearing part of the transmitted signal. An example transmitter appropriate for such constant-envelope modulation schemes is referred to as a translational loop transmitter. In this transmitter architecture, the digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to produce an intermediate frequency (IF) carrier fIF signal. The resulting digital signal is converted to analog using a digital-to-analog converter (DAC) and a low pass filter (LPF) that filters out undesired digital images of the IF signal. A phase locked loop (PLL)then translates, or shifts, the IF signal to the desired RF frequency channel and a power amplifier (PA) delivers the appropriate transmit power to the antenna.
According to related art, a radio transmitter includes a digital processor coupled to receive digital baseband data and produces a digital waveform characterized by an intermediate frequency and a phase. The output of the digital processor is produced to a DAC that is capable of processing intermediate frequency digital data while avoiding unnecessary quantization noise to produce an analog outgoing signal to a low pass filter. The low pass filter removes harmonics of the analog output of the DAC and produces an outgoing low pass filtered signal to a translational loop that up-converts the analog signal from an intermediate frequency to a radio frequency. The phase information originally produced by the digital processor is maintained in the RF signal produced by the translational loop to a power amplifier for amplification and radiation from an antenna.
The described radio transmitter is simplistic and is intended to represent various embodiments of RF transmitters, including embodiments in which the processing described occurs for both in-phase and quadrature phase signal paths (I and Q signal paths, respectively).
One typical radio transmitter used for GSM cellular telephony includes a digital processor that delivers a phase signal, θ(t), to the transmitter for further processing and RF transmission. The transmitter is typically a digital baseband processor that performs the necessary pulse shaping, modulation, and interpolation filtering, followed by in-phase and quadrature digital-to-analog converters, low pass reconstruction filters, and analog baseband mixers. A summing node combines the mixer outputs that are followed by low pass filtering. The remaining components of the transmitter are a phase and frequency detector (PFD), 26 MHz crystal reference (X-TAL), a charge pump (CP), a loop low pass filter (LOOP FILTER), a voltage controlled oscillator (VCO), a pair of offset mixers, as well as appropriate low pass filters. RF channel selection is achieved by employing a Fractional N frequency synthesizer. Within the translational loop, a sum of the mixing products of the baseband I & Q components is low pass filtered with down-converted RF output I & Q and is phase compared with a 26 MHz clock to generate a 26 MHz sinusoid whose excess phase component equals the difference between the desired baseband phase signal and the RF output phase signal. The 26 MHz carrier is extracted by the PFD whose output is the phase error signal.
With proper PLL design, the closed loop tracking action causes the error signal to approach zero; hence, the phase of the RF output carrier from a voltage controlled oscillator is produced at 900 MHz which tracks the phase of the baseband signal, as desired.
Other types of digital communication schemes, such as the 3π/8 offset, 8-level PSK employed in the EDGE cellular telephony standard, have non-constant envelope and the transmitter signal, sRF(t), can therefore be represented in quadrature form assRF(t)=i(t)cos(2πfct)+q(t)sin(2πfct),   (2)or, equivalently, in polar form assRF(t)=r(t)cos(2πfct+θ(t)),   (3)where both r(t) and θ(t) are information-bearing components of the transmitted signal. The signal components r(t) and θ(t) are referred to as the envelope and phase of sRF(t), respectively.
Transmitters in which a phase path is formed separate from an envelope path, as described above, are known as polar transmitters. One problem with polar transmitters, as will be described below, is delay mismatch between the phase and envelope components for the signals in the envelope and phase signal paths.
Delay mismatch may cause the translational loop output to be modulated by a time shifted envelope signal relative to the phase signal thereby causing an error in the transmitted RF signal. It should be noted that only delay mismatch between the envelope and phase signal paths has a detrimental effect on the transmitted signal. Any common delay along the envelope and phase signal paths does not affect the quality of the transmitted signal. One reason delay mismatch is problematic is that delay mismatch causes a phenomenon known as spectral re-growth, which results in an elevated power spectrum that may violate spectral mask requirements.
FIG. 1 is a diagram that illustrates delay mismatch between signals that are expected to be synchronous. In polar transmitters, for example, a phase signal component is desirably expected to arrive simultaneously with a phase signal to amplitude modulate the phase signal. FIG. 1, therefore, provides one example of a delay mismatch in which a phase signal is delayed relative to an envelope signal by a slight amount, shown as dt. For example, FIG. 1 may represent a phase signal that is delayed relative to an envelope signal in a polar transmitter by a value of 20 nano-seconds. Ideally, in a polar transmitter, such phase and envelope signals should be aligned for proper operation of the polar transmitter. One problem that is observable for such delay mismatch is that of spectral re-growth resulting in sustained spectral mask violations in adjacent channels.
FIG. 2 is a diagram that shows several example RF signal output power spectra corresponding to an Enhanced Data rate GSM Evolution (EDGE) signal with various delay mismatches between the envelope and phase signal paths of the RF polar transmitter. FIG. 2 illustrates values of adjacent channel power due to delay mismatch for adjacent channels 1, 2 and 3 (first, second and third adjacent channels) for frequency offset values of 0 Hz to 1 MHz. As may be seen in FIG. 2, the significant impact of delay mismatch, for example with a delay mismatch of 80 nS or greater, is that the spectral mask requirement of −54 dBc at 400 kHz offset is not satisfied. As may be seen, adjacent channels to a channel carrying a primary signal experience a corresponding spectral power growth or influence from delay mismatch in the primary channel. Such adjacent channel power growth due to a primary channel delay mismatch may be evaluated in terms of an Adjacent Channel Power Ratio (ACPR). Such channel power growth is undesirable because design/standards-based spectral mask requirements may be violated. In order to restore the RF transmitter performance, delay mismatch must therefore be substantially canceled or reduced to reduce a value of ACPR if the ratio is calculated with the adjacent channel power in the numerator and the primary channel power in the denominator of the ratio. The delay mismatch may be reduced or canceled by introducing one of a positive or negative delay of a signal in one of the paths.
It is clear from the foregoing description of the related art, that a method and an apparatus are needed that addresses the problems of delay mismatch and that finds a method for minimizing adjacent channel power ratios as defined herein while adhering to spectral mask requirements to improve performance of polar transmitters that are presently being designed.